This invention relates to a is for obtaining in large volume a gas stream that is 90%-99% and higher by volume in one component of a gaseous mixture. This invention especially relates to an adsorption process for providing a gas stream enriched in at least one component by means of a pressure swing adsorption system using carbon molecular sieves. More particularly, this invention relates to a method for providing an inexpensive and high volume source of gases such as nitrogen, hydrogen or methane, requiring less energy to operate than either cryogenic or other pressure swing adsorption systems, and yet supplying gases of comparable quality.
The term gaseous mixture, as used herein, refers to air and other gas mixtures primarily comprised of two components of different molecular size. The term enriched gas refers to a gas comprised of the component(s) of the gaseous mixture relatively unadsorbed after passage of the gaseous mixture through an adsorbent that meets a predetermined purity of, for example, from 90 to 99% in one component. The term lean gas refers to that gas exiting from an adsorption zone that fails to meet the purity standard set for the enriched gas. In the case of a two (or more) column adsorption zone, lean gas is that gas passing solely through the last column comprising the zone as well as gas that fails to meet the purity standard set for enriched gas.
A gaseous mixture may be fractionated, or separated, using pressure swing adsorption by passing the mixture at an elevated pressure through a bed of adsorbent which is selective in its capacity to adsorb one or more of the components of the mixture. This selectivity is governed by the pore size distribution in the adsorbent and the pore volume of the proper pore size for adsorption of a particular gas component. Thus, gas molecules with a kinetic diameter less than or equal to the pore size are retained, or adsorbed, on the adsorbent while gas molecules of larger diameters pass through the column. The adsorbent, in effect, sieves the gas according to its molecular size. The gas mixture may also be fractionated because of different rates of diffusion of its components into the pore system of the adsorbent.
As the gas travels through the adsorbent column, the adsorbent pores are filled with gas molecules. One can envision an adsorption front, moving through the column, akin to the liquid adsorption front moving through a solid adsorbent in a column chromatography system. After some time the gas exiting the column is essentially the same in composition as the gas that entered the column of the adsorbent. This is known as the breakthrough point. For example, a gaseous mixture enters the adsorbent, and the identical mixture exits the adsorbent. At some time prior to this breakthrough point, the adsorbent must be regenerated.
After treatment of the gas mixture to adsorb selected components therefrom, the flow of the gaseous mixture through the column is interrupted and the adsorbent is regenerated by purging it of the adsorbed components either by vacuum or by passing through the column, generally in the opposite direction of flow taken by the gaseous mixture (i.e., countercurrently), a purge gas stream which may comprise a portion of the purified product.
Pressure swing adsorption usually includes at least two columns of adsorbent so that while one column is being regenerated, the other is in the adsorption phase producing product gas, that is, the columns interact as two alternating adsorption zones.
More than one column may be employed in an adsorption zone. When more than one column comprises such a zone, the columns may be connected in either a serial or a parallel arrangement. When adsorbent columns are connected in series, gas exiting one column of the zone enters the inlet end of the next column comprising the zone. In a parallel arrangement, the gas mixture enters the inlet end of all columns comprising the zone. Generally, a serial arrangement is preferred for high purification, while a parallel arrangement allows for large quantities of product gas in a short time cycle. As used herein, the term adsorption zone is understood to refer either to a single column zone or a serially arranged two column zone. In either case, each adsorption zone has an inlet end, for example, the inlet of a single column zone is that one column's inlet end, and for a two column zone (arranged in series) the inlet end is at the inlet of the first column in the zone. Each adsorption zone has a corresponding outlet end. For the two column zone, this is at the outlet end of the second column comprising the zone. When using two adsorption zones, by cycling (alternating) between these adsorption zones, product gas is obtained constantly.
A known process is the use of carbon molecular sieves for the production of enriched nitrogen from air. See for example Vesterdal, U.S. Pat. No. 2,556,859 and Munzner et al., U.S. Pat. No. 3,960,522. These sieves possess a pore structure with a size comparable to the kinetic diameter of oxygen. When used in a pressure swing adsorption system, these sieves selectively adsorb oxygen from a gas mixture, allowing other components especially nitrogen to pass.
A four column pressure swing adsorption unit has been successfully employed in the separation of hydrogen gas from its mixture with carbon dioxide, water and light aliphatic hydrocarbons. See for example, Wagner in U.S. Pat. No. 3,430,418.
Also known is the fractionation of other binary gas mixtures by pressure swing adsorption. For example, carbon monoxide from its mixture with hydrogen using zeolite 13X and carbon dioxide from its mixture with fuel gas mixtures using charcoal, alumina or silica. See, Simonet, U.S. Pat. No. 3,884,661.
Binary gas mixtures of argon and oxygen or helium and methane have been separated on an adsorbent of partially oxidized carbon in a pressure swing adsorption process. See, German Auslegungsschfrift No. 2,045,200.
Typical problems in the present pressure swing adsorption and carbon molecular sieve technologies include; low yield of product gas, large amounts of molecular sieve required and energy inefficient regeneration methods.